It is well known to provide a motor vehicle occupant restraint system having a shoulder belt which extends over the shoulder and across the chest of the seated occupant. An inertia responsive seat belt retractor is conventionally associated with one end of the shoulder belt and has a windup spring which rotates a reel to wind or retract the belt to a taut condition across the chest of the occupant. The inertia retractor permits belt unwinding rotation of the reel to extend the belt as the occupant leans forwardly to reach the vehicle operating controls and locks the belt against extension in response to an acceleration stimulus experienced by the vehicle body.
It is also known to provide a retractor having a winding prevention mechanism which enhances occupant comfort by locking the reel against belt winding rotation by the windup spring to hold the belt at a set length establishing a slackened condition about the occupant. U.S. Pat. No. 4,023,746, issued to Joseph J. Magyar on May 17, 1977, provides an inertia locking retractor having a winding prevention mechanism which is selectively actuated and deactuated in response to a predetermined sequence of reel rotation induced by movement of the restrained occupant.
It is also known to provide a passive belt system having one end of the belt mounted on the door, the other end of the belt mounted on the vehicle body inboard the seat, and a retractor associated with one of the belt ends for winding the belt to a taut position about the occupant upon closing movement of the door and unwinding the belt upon opening movement of the door so that the swing geometry of the door stows the belt forwardly of the occupant.
It would be desirable to employ a winding prevention retractor in a door mounted passive seat belt system to enhance occupant comfort. However, the Magyar type winding prevention mechanism which is controlled in response to the predetermined sequence of belt winding and unwinding could be subject to unintended actuation of the winding prevention mechanism upon inadvertent random oscillatory movement of the door in the open position so that the belt would not be wound to the restraining position about the occupant upon a subsequent closure of the door until the occupant intentionally cancels the winding prevention mechanism by unwinding the belt from the reel.
The present invention features a new and improved winding prevention mechanism having a mechanism for measuring the extent of belt unwound from the retractor and adapted to block engagement of the winding prevention pawl whenever the belt is unwound beyond the extent consistent with deployment of the belt in the normal occupant restraint position so that the winding prevention pawl cannot become engaged when the door is opened and the winding prevention pawl will be automatically disengaged when the door is opened if the pawl has been previously set.